In recent years, such problems as deteriorating greenhouse effect are grievous challenges for environment and sustainable economic development. Hence, technologies for decreasing emission of carbon dioxide which contributes to greenhouse effect and global climate change become focus of interest of various countries. Some countries, such as the United States, have proposed a method for decreasing emission of carbon dioxide-carbon dioxide capture technology, which is currently aimed mainly at capture of carbon dioxide emitted from power plants.
The emission of carbon dioxide from power industry in the world now accounts for 40% of the total emission of carbon dioxide on the earth. The global power generation capacity is predicted to be doubled in 2030. If no effective measure is taken, the emission of carbon dioxide will be increased by ⅔ accordingly. In China, where thermal power generation plays a predominant role, capture and storage of carbon dioxide in thermal power plants are particularly important for reducing emission of greenhouse gas since the carbon dioxide emitted from thermal power plants originates mostly from combustion of coal.
At present, there are mainly three carbon capture technologies under intensive development, namely post-combustion decarbonization, pre-combustion decarbonization and oxygen-rich combustion, wherein the pre-combustion capture technology can only be used in newly built power plants, while the other two technologies can be used in both newly built ones and existing ones.
Post-combustion decarbonization separates carbon dioxide from flue gas. Methods for collecting carbon dioxide mainly include chemical solvent absorption, adsorption, membrane separation and the like. The best collection method at present is absorption using amine as the chemical solvent. Amine reacts with carbon dioxide to form a salt compound. Then, the solvent is warmed up, and the compound decomposes to separate out the solvent and highly pure carbon dioxide. Since the flue gas generated in combustion contains a variety of impurities, the existence of which may increase capture cost, the flue gas needs pretreatment (water scrubbing, cooling, dewatering, electrostatic dedusting, desulfurization, denitration, etc.) before absorption to remove active impurities (sulfur, nitrogen oxides, particles, etc.) therein. Otherwise, these impurities will preferentially react with the solvent, leading to consumption of a great deal of solvent and corrosion of equipments.
Due to aerosol particles such as water entrained in flue gas, the amine solution in an absorption tower will be diluted gradually, leading to increased consumption of amine solution, and amine solution rich in carbon dioxide is discharged, resulting in discharge of highly concentrated organic pollutants which are difficult to degrade. On the other hand, these aerosol particles tend to induce foaming of amine solution in the absorption tower, and thus render loss of amine solution. As a result, amine solution is entrained in the gas discharged from the absorption tower, which further increases the consumption of amine solution and causes atmosphere pollution. A pneumatic cyclonic separation method and a device corresponding thereto may be used to control the amount of dust and liquid drops entrained in flue gas. However, the existing pneumatic cyclonic separation technology uses a drop pressure of 500-600 Pa, or even up to over 1000 Pa, and thus can no longer be applied in a system which is located after a water scrubbing tower and before a flue gas absorption tower. Otherwise, the pressure of flue gas will be insufficient, and a pressurizing system will be needed for flue gas, which is quite uneconomical. In order to control the content of aerosol in flue gas after dust scrubbing, the present invention uses a rotary flow separation technology with a lower pressure drop, i.e. a rated pressure drop of 20 mm H2O column, namely a pressure drop of 200 Pa, which is ⅓-⅕ of the pressure drop of an existing pneumatic cyclone. This can not only effectively control and capture the flue gas moisture and dust that enter an absorption tower, but also efficiently utilize the remaining pressure of the flue gas with lower partial pressure.
If the solid content in the flue gas and recycled scrubbing water that enter a water scrubbing tower is higher than 100 mg/kg, the water scrubbing tower will be clogged, resulting in shorter continuous running cycle and frequent reverse flushing. According to existing measures, the dust in flue gas is removed using a pneumatic cyclone before the flue gas enters a water scrubbing tower, and the solid in recycled scrubbing water is deposited in a water scrubbing tank. However, the flow rate of flue gas in thermal power plants is large. For example, the flue gas from a thermal power generation facility has a flow rate of 630000 Nm3/h, for which a high-precision pneumatic cyclone has to be selected to capture the aerosol particles having an average particle size of about 4 microns in the flue gas. Thus, the pneumatic cyclone has a huge profile size and a high construction cost. In addition, the overall pressure of the flue gas is only 2000 Pa. If an existing pneumatic cyclone is selected, 600-1000 Pa will be used, and subsequent procedures will suffer from tight pressure supply. A pneumatic cyclone is not used in the invention to implement gas-liquid separation of flue gas before it enters a water scrubbing tower. Instead, flue gas is allowed to enter a water scrubbing tower directly, and the effluent water from the water scrubbing tower is subjected to liquid-solid microcyclonic separation, or the recycled scrubbing water is subjected to microcyclonic separation and purification. Additionally, since a great deal of recycled water is needed to be scrubbed due to the enormous amount of flue gas to be treated, a technical solution combining partial microcyclonic separation and multi-stage microcyclonic concentration of sludge is used in the invention, which can ensure not only no lowering or minute lowering of the remaining pressure of flue gas, but also clearance and enrichment of PM2.5 particles in the flue gas.
In an existing flue gas carbon dioxide capture system, after carbon dioxide in flue gas is absorbed and captured by a carbon dioxide absorbent in an absorption tower, a tail gas of the carbon dioxide capture system is generated. The tail gas entraps a lot of aerosol particles, particularly PM2.5 particles, which, if not captured and removed before the tail gas is emitted to atmosphere, will render a severe hazard to atmospheric environment as well as entrainment loss of scrubbing liquid. This will increase the running cost of the whole set of flue gas carbon dioxide capture facility to a large extent, and influence the economic efficiency greatly. Therefore, a momentous technical problem in the field of energy and environment is how to recover these aerosol particles. An existing method for capturing aerosol particles involves a rotary flow separator or an electrical trap, among which the pressure drop of the former is generally 500-600 Pa, or even up to over 1000 Pa. This existing technology can not be used in a flue gas carbon dioxide capture system for the following reasons. First, the pressure drop is too high, and the tail flue gas from the top of an absorption tower can not sustain such high consumption of pressure drop. Second, if an existing cyclone technology is to be selected, the tail flue gas has to be pressurized, which is complex and uneconomical. With regard to an electrical trap, hidden safety trouble exists due to high moisture content in tail flue gas. In addition, the cost of an electrical trap is high, and there is no appropriate and economical electrical trap in the industry. Therefore, a micro-size rotary flow separator with low pressure drop and high separation accuracy has to be selected.
In a flue gas carbon dioxide capture system, mixed amine solution particles are entrained in regeneration gas carbon dioxide and may have negative impact on subsequent devices and processes, even leading to environmental pollution. Another issue is the loss of aerosol particles. For example, a carbon dioxide carbon facility on a scale of 1 million tons/year consumes about 1600 tons/year of mixed amine solution solvent annually at a price of 40 million yuan (RMB). 80% of the mixed solvent that has been consumed is taken away by purifying gas in the form of aerosol, particularly PM2.5 particles. How to recover these aerosol particles is a great technical challenge in the field of energy and environment. An existing method for capturing aerosol particles involves a rotary flow separator or an electrical trap, among which the pressure drop of the former is generally 500-600 Pa, or even up to over 1000 Pa. This existing technology can not be used in a flue gas carbon dioxide capture system for the following reasons. First, the pressure drop is too high, and the flue gas having low partial pressure can not sustain such high consumption of pressure drop. Second, if an existing cyclone technology is to be selected, the flue gas has to be pressurized, which is complex and uneconomical. With regard to an electrical trap, hidden safety trouble exists due to high moisture content in regeneration gas. In addition, the cost of an electrical trap is high, and there is no appropriate and economical electrical trap in the industry. Therefore, a micro-size rotary flow separator with low pressure drop and high separation accuracy has to be selected.
For capture of flue gas carbon dioxide, Research Institute of Nanjing Chemical Industry Group has developed a new technology for capturing low-partial-pressure (flue gas, etc.) CO2, wherein CO2 is captured using a mixed amine solution consisted of aqueous MEA solution as the bulk and active amine, antioxidant and erosion retarder as additives, and the problems of high loss of amine due to degradation, serious erosion of equipments, high consumption of energy, etc., have been solved thereby. But in long-term operation, plenty of solid particles and engine oil are entrained in the mixed amine solution. If they are not removed before the mixed amine solution enters an absorption tower, downstream lines and devices will be clogged unavoidably. Not only the efficiency of carbon dioxide capture will be deteriorated, but also the whole set of facility will likely be paralyzed. At present, an activated carbon filter is generally disposed in front of an absorption tower to remove solid particles and engine oil entrained in mixed amine solution. However, the operational principle of an activated carbon filter is adsorption of impurities into the activated carbon particles. The filter has good initial adsorption effect, but its adsorption capacity will inevitably decrease to certain degree after a long period of time, and its adsorption effect will decrease as well. As a result, the activated carbon in the activated carbon filter has to be scrubbed or replaced periodically, which influences the safe, steady and efficient operation of the whole set of flue gas carbon dioxide capture facility. Liquid-solid microcyclones and liquid-liquid microcyclones in prior art are capable of removing solid particles and engine oil entrained in mixed amine solution efficiently, with the separation accuracies for solid particles and oil drops being 3 microns and 10 microns respectively. Moreover, the filter is resistant to clogging and needs no replacement of inner members. It is thus suitable for long-term steady use.